In a watch, the calendar mechanism allows the date of the month to be indicated by means of a hand moving over the dial or by means of a disk rotating under the dial and showing its information through a window. This kind of system is well known to those skilled in the art and is described in detail, for example, in the work entitled “Théorie de l'horlogerie” by Reymondin et al., Fédération des Ecoles Techniques, 1798, ISBN 2-940025-10-X, pages 189 et seq.
It will be recalled that the calendar indicator, whether a disk or a hand, is actuated off the hours wheel, via the calendar gear-train the last element of which is a thirty-one wheel performing one revolution in thirty-one days by advancing by one step of 360/31° every twenty-four hours, at around midnight. In the case of a display using a hand, the hand is mounted so that it rotates as one with the shaft of this wheel. In the case of a display using a disk, this disk is driven by a finger integral with the thirty-one wheel.
For months comprising fewer than thirty-one days it is necessary, in the case of simple calendars, to make a manual correction. This operation can be performed either by rotating the winding stem in its quick date-setting position or by actuating a push-button fitted freely into the watch case middle. Of course, the operation has to be performed on the last day of a month comprising fewer than thirty-one days or on the first day of the next month.
In order to avoid the user having to perform such updates, various mechanisms, known as perpetual calendars, have been developed. They automatically adapt to the length of the months, sometimes even predicting leap years. A toothed wheel, known as a cam, has hollows the depths of which are correlated to the length of the months. Schematically speaking, the cam comprises forty-eight or twelve sectors, depending on whether or not the mechanism takes account of leap years. A complex assembly of several other cams and levers, one of which is equipped with a nose collaborating with the first cam, transmits to the thirty-one wheel the order to jump by one, two or three days at the end of a thirty-day, a twenty-nine-day or a twenty-eight-day month, respectively.